A Low-Noise High-Output CapacitorMicrophone System

RICHARD S. BURWEN

LexingtonMA 02173

The dynamic range of a capacitor microphone has been considerably extended by means ofan advanced FET feedback amplifier. This microphone delivers +20 dBm into 600 ohms atmaximum sound pressure levels of 110, 125, or 140 dB. Its A-weighted noise level of 15dBis contributed mostly by the capsule instead of the amplifier.

INTRODUCTION: Now that we have an audio signal ing the microphone for a line level output of 20 dBm intoprocessing system [1] capable of extending the dynamic 600 ohms, cable noise pickup has been rendered insig-range of a studio tape machine to 110 dB, the.previously nificant, and the need for a low level console preamplifieradequate noise levels of studio consoles and microphones with its attendant noise has been eliminated.have become too high. The extended dynamic range Many recording studios use a wide variety of mi-capacitor microphone system to be described is designed crophone types in a single recording session because theto solve this noise problem and, at the same time, increase sounds of certain microphones are considered optimum forthe maximum acoustic input capability, the sound sources with which they are used. In the

In recording, noise is most apparent in a multitrack author's recording system the same wide dynamic, rangemixdown and when microphones are picking up at a microphone serves all purposes. In this case the sound isdistance. The noise levels of today's high-quality changed by means of an extremely flexible wide dynamiccapacitor microphone systems are in the range of 20-30 range program equalizer described in an earlier paper [2].dB sound pressure level. Although this noise may be Directional patterns can be selected by using eithersomewhat comparable with the noise of a well constructed pattern-switchable capsules providing either two or three

studio, the room noise does not mask the microphone directional patterns or by separate capsules having thenoise because room noise occurs primarily at low frequen_ desired directional patterns. Sufficient gain for close,cies and the audible microphone noise is a hiss at high medium, or distant miking is attained by means of a gainfrequencies, switch on the microphone itself which provides 20 dBm

In this new microphone system the noise level has been: output for sound pressure level inputs of 140, 125, or 110extended downward to 15 dB sound pressure level dB, respectively.A-weighted. Its maximum input signal has been extended The following sections will describe the sources of

upward to 140 dB sound pressure level without taking the noise, how the electronic system is designed, and itsmicrophone apart and adding a capacitive pad. By design- performance characteristics.

278 JOURNALOFTHEAUDIOENGINEERINGSOCIETY

NOISE SOURCES polystyrene or other low-loss insulation which takes careof this problem.Fig. 1 shows the principal electronic noise sources in

the capacitor microphone system. The desired signal isgenerated by the variable microphone capacitance which AMPLIFIER AND POWER SUPPLIESis biased by high voltage. In this particular microphone thecapacitance is approximately 43 pF, which is fairly high A block diagram of the model 3000 low-noise capacitorfor a capacitor microphone. One of the principal sources microphone system is shown in Fig. 2 and the schematicof noise is the voltage noise EN4 generated within the diagram in Fig. 3. The system is designed around an

amplifier Al. Also the amplifier has an input current noise existing microphone capsule, the Schoeps MKT-45, 60-VIm which develops a voltage across the source capaci- series, which was chosen for several reasons. This capsuletance. Another factor which influences the signal and has a pleasing subjective quality, excellent frequencytherefore influences the signal-to-noise ratio is the input response, a high capacitance which attenuates input noisecapacitance of the amplifier system. This loads the mi- currents previously described, and a fairly high output.crophone capacitance and attenuates the input signal, Another importan/ advantage is the three-wire floatingthereby making the voltage noise of the amplifier more capacitance arrangement which permits a feedback volt-

age to be added into the input by feeding it in series withsignificant relative to the microphone signal, the capsule. The capsule has been manufactured withHum pickup due to stray capacitance to an ac source EN,can be very serious if the microphone is not tightly single or switchable directional patterns, including omni-

directional, bidirectional, cardioid, and hypercardioid.shielded. Similarly stray capacitance to any noise source The system consists of an input preamplifier having twosuch as a power supply lead or even a zener diode can,introduce significant noise into the microphone capaci- high transconductance field effect transistors (FET) intance. Furthermore, capacitance to any dc source consti- parallel, another low-noise amplifier, and a final outputtutes an unwanted capacitor microphone which must be amplifier. This output amplifier is dc coupled to the loadand delivers 7.7 volts rms into 200 ohms or more.rigidly supported and adequately damped mechanically toprevent vibration from altering the system frequency Feedback around the entire system comes from the output

through an attenuator to the low side of the capsule. Toresponse, stabilizethe dc outputlevel, additionalfeedbackat sub-Perhaps one of the largest sources of noise in capacitormicrophone systems is the load resistor used for biasing audio frequencies, amplified 80 dB in a low-frequencythe capsule. In a typical capacitor microphone system this amplifier, is delivered to the 20 000-megohm capsule loadresistor to bias the FETs.resistor is about 250 megohms. This value is so low that it Power is supplied from an external -+15-volt sourcedelivers an appreciable noise current into the microphone using ordinary regulated supplies. Ripple and noise arecapacitance from its thermal agitation noise generatorEma, attenuated by additional low-noise regulators within theBy increasing the value of this resistance the current microphone which provide +12 volts and -11 volts fordelivered into the microphone capacitance can be reduced the low-level amplifiers. A +60 volt dc-dc converter isby the square root of the resistance ratio. In the new

used to bias the capsule and is also powered from themicrophone design one of the principal means of reducing - 11- V regulator.noise is an increase in this load resistance to 20 000

In the FET preamplifier, noise is minimized by usingmegohms, low-capacitancehigh-transconductanceFETs operatingat

Another type of noise that has been troublesome in approximately 14 000 micromhos. With a short-ci_uited

testing microphone amplifiers is a type of leakage current input the equivalent input noise voltage in the upper audiooccurring, for example, within or on the surface of silver bandwidth is approximately 1/3 microvolt. Using high-mica capacitors used to substitute for the microphone transconductance FETs and a 20 000-megohm gate resist-capsule. Leakage or dirt can generate quite a bit of noise, or results in an input leakage problem, and it may require aand considerable variation in noise occurred just from fairly substantial voltage delivered to R1 to offset thisusing different types of capacitors. In testing this system a leakage current and properly bias the FETs. This voltageTeflon capacitance eliminated most of this type of noise. is supplied by the 80-dB low-frequency amplifier whichMost capacitor microphone capsules are insulated with

senses the output dc offset, provides 80 dB of very-low-frequency gain, and can deliver from 0 to -9 volts into

E_2 resistor RI. This voltage is sufficientto overcomea

.,t -_)-L leakage current of 400 pA. The amplifier provides no

L t s'_^_ __Ni_ feedback within the audio range, but only at direct current.E_

Because of its high gain the output dc offset of the entire_NPU, system is determinedby the inputdc offset of the 80-dBMICROPHONE

_3PFFNi__PF ' _CAP^C,_C_ low-frequency amplifier which is trimmed to within -+3FEEOB^c_"_'_t F._ millivolts.

DIVIDER _' _R2 : = Power supply noise, as mentioned, is important, and sothe powerfor the first stage comesfrom a +12-voltregulator which has only 0.5 microvolt of noise output.

Fig. 1. Noisesources. The attenuation of noise from the extemal +15-volt

MAY 1977, VOLUME 25, NUMBER 5 279

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supplyis 100dB. lengthwiseprinted circuit board mounted within a pair ofNot only must the FETs and power supply he designed concentric brass tubes approximately 10.4 inches (0.26 m)

for low noise but also the second amplifier must be a long and 0.8 inch (20 mm) in diameter. The tubes arelow-noise type for two reasons. One is the rather low gain insulated from one another and constitute the inner andof the first FET stage which reflects every microvolt of outer shields. Figs. 4 and 5 show the finished unit and itsnoise in the second amplifier back to the input as 0.12 interior construction.microvolt. The main reason is that the gate to drain

capacitance of the FETs feeds back into the capsule a MEASURED pERFORMANCEnoise voltage equal to a fraction of that appearing on thedrains. Therefore the second amplifier has been designed The output noise level of the microphone systemfor only 0.5-microvolt input noise. This amplifier is measured in one-third octave bands is plotted in Fig. 6.powered by both the +12-volt low-noise regulator and the Curve C shows the spectrum for the amplifier alone when-11-volt low-noise regulator. The -11-volt regulator has connected to a 40-pF source capacitance. Curves A and Bapproximately 2-microvolts output noise and attenuates show the noise levels produced with an omnidirectionalripple and noise from the _15-volt external supply by 90 cardioid two-way switchable capsule mounted. ThesedB. curvesweremeasuredby insertingthe microphoneintoa

The low-noise amplifier feeds a unity gain dc output soundproof metal tank lined with absorptive material.amplifier designed to provide the high output current They show that the major contributor of noise is thecapability along with low open-loop distortion and short- capsule itself which has some 6 to 12 dB more noise thancircuit protection. The overall gain of the amplifier system the amplifier. In the capsule, noise may be contributed byis determined by the feedback from this output amplifier the thin layer of air between the diaphragm and its backthrough an attenuator that feeds the low side of the plate. However, the mechanical venting used to changemicrophone capsule. The attenuator has three switch the directional pattern has a considerable effect as shownpositions, 10, 25, and 40 dB, which determine the gain of by curve A for the cardioid pattern which is as much as 5the system from the capsule. These positions provide the dB above the omnidirectional pattern in the vicinity of 300full +20-dBm output at sound pressure levels of 140, 125, Hz.or 110 dB, respectively. Within the attenuator there is alsoa feedback equalizing network which boosts the systemfrequency response 2 dB at 20 kHz to partially compensatethe natural high-frequency rolloff of the microphonecapsule. Although the bandwidth of the feedback loopextends to 8 MHz at minimum gain, capacitive loads areisolated at high frequencies so that excellent loop stabilityis maintained with cable capacitances as high as 0.01 /xF.

Bias for the capsule is produced by a +60-volt dc-dcconverter which is unconventional in that it is completelyshielded. It operates just below 500 kHz and has decou-pling on the input power supply leads as well as goodoutput filtering. The leakage from this power converterinstead of being tens of millivolts is only a few microvolts.Its noise level is filtered at its output down to less than 0.1microvolt, and the power converter has no effect on thesystem noise.

The output signal comes from a five-pin audio connec-tor which is used to receive the input power as well. Notethat this system has a single-ended output, but it can feedeither a single-ended input amplifier at the far end of thecable, if the cable is short, or preferably a differentialinput amplifier. The entire system is double shielded inorder to reduce the effects of hum pickup and RF pickup ,from switches on power lines and from light dimmers. Theamplifier is contained within an inner shield which is the Fig. 4. Low-noise capacitor microphone.basic ground for the system, and the outer shield iscompletely isolated and grounded at a single point back atthe studio console. The only portion of the inner shieldthat is exposed when handling the microphone is the

capsule itself. To eliminate noise pickup the capsule has tobe mounted on the inner shield rather than on the outer

shield. Fig. 5. Amplifierand regulatorassemblyinside theMost of the electronic circuitry is contained on a microphone.

282 JOURNAL OF THE AUDIO ENGINEERING SOCIETY

A LOW-NOISE HIGH-OUTPUT CAPACITOR MICROPHONE SYSTEM

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ao so ,00 2_0 _*o'.... _ "i_ -_'_'""""_'_ 00_ A new low-noise series feedback amplifier has extended"' the dynamic range of an. existing capacitor microphone

capsule to its full capability of 125 dB. Used in conjunc-Fig. 6. Noise spectrum measured in one-third octave bands, tion with a flexible equalizer the microphone system is

A--two-way capsule switched to cardioid; B--same capsule capable of performing all the tasks for which studiosswitched to omnidirectional; C--amplifier noise with 40-pFsource, normallyuse severaldifferenttypesand brandsof mi-

crophones simultaneously. Its high output of +20 dBmMicrophone noise is generally measured with one of eliminates the need for a low-level microphone pream-

several weighting filters that attenuate below 600 Hz and plifier with its dynamic range limitations and permits

above 10 kHz. Using an A-weighting filter the equivalent connection directly to a line level input. In many in-input noise for the omnidirectional pattern is typically 15 stances, either the microphone or the microphone plusdB sound pressure level. Using a flat response filter with equalizer can feed directly a 110-dB dynamic rangecutoff at 20 Hz and 20 kHz, the noise increases by recording system, thus circumventing the studio console.approximately 7.5 dB to 22.5 dB sound pressure level. In multitrack recording the wide dynamic range andWhile the noise output, when switched to the cardioid accuracy of these new tools raises a new standard ofpattern, is 2 to 2.5 dB higher, the ·overall acoustic performance and reduces the effects of operator error.signal-to-noise ratio is about the same because the on-axis Future development in capsules, if directed toward reduc-sensitivity of the microphone increases by about 2 dB ing acoustic noise, might extend the dynamic rangewhen switched from omnidirectional to cardioid. Referred downward by another 6 dB.

to the microphone capsule, the amplifier electrical noiseA-weighted is typically 0.56 microvolt and wideband is REFERENCES

1.5 microvolts. Harmonic distortion due to the amplifier [1] R. S. Burwen, "Design of a Noise Eliminator Sys-varies with the gain setting, and the load but is at worst tem," J. Audio Eng. Soc., vol. 19, pp. 906-911 (Dec.0.05% when delivering 7.7 volts rms into 200 ohms. The 1971).frequency response measured by inserting a voltage in [2] R. S. Burwen, "A Wide-Dynamic-Range Programseries with the low side of the capsule is up a maximum of Equalizer," J. Audio Eng. Soc., (Project Notes/Engineer-0.1 dB at 20 Hz and is up nominally 2 dB at 20 kHz. The ing Briefs), vol. 23, pp. 722-726, (Nov. 1975).

THE AUTHOR

Richard S. Burwen received the S.B. and A.M. degrees supplies, space vehicle equipment, television, automo-from Harvard University in 1949 and 1950, respectively, tire, airborne equipment, laboratory instruments, linear

He was involved in circuit design at Bell Telephone integrated circuits, and audio. He holds a number ofLaboratories, Spencer-Kennedy Laboratories, Krohn-Hite patents and is the author of numerous papers on audio andCorporation, and Honeywell, Inc., until 1961. For the analog circuits. At the same time Mr. Burwen was one ofpast fifteen years Mr. Burwen has. been an independent the founders of Analog Devices, Inc., and Ohmtec Corpo-circuit design consultant for over fifty companies in the ration. He is currently technical consultant to Burwenareas of industrial control, medical electronics, power Research, Inc., and several other companies.

MAY1977,VOLUME25,NUMBER5 283